In a possible terrorist attack Wednesday afternoon, a vehicle rammed into a crowd of pedestrians outside the British Parliament in London. A man then exited the vehicle and stabbed a police officer to death; other officers then shot and killed the assailant.

"Four people have died," said Britain's top counter-terrorism officer, Mark Rowley. "That includes the police officer who was protecting Parliament and one man we believe to be the attacker, who was shot by a police firearms officer."

The attack, which police are treating as terrorism, comes exactly one year after the series of suicide bombings in Brussels that killed 32, along with three bombers. London will join several other prominent European cities, including Paris, Berlin and the aforementioned Brussels as a victim of recent incidents of terrorist attacks, if in fact that turns out to be the case, as seems likely.

It all began when the car drove over Westminster Bridge, which leads to the Parliament building. Dozens of pedestrians on the bridge were either hit or barely avoiding the vehicle, with some even jumping into the Thames River below to escape. Once over the bridge, the car crashed into a railing, after which the suspect attacked a police officer, fatally stabbing him. After that, police came running to the scene and killed the man.

“This is the day we have planned for but we hoped would never happen,” Mr. Rowley said. “Sadly, it’s now a reality.”

The British Prime Minister, Theresa May, was evacuated while Parliament was locked down, with many members of Parliament hiding inside as gunshots rang out outside.

It appears now that the attacker was alone, though the investigation is still in its earliest stages.